Bless create-and-switch running the full switch sequence

Settled from the Redesign Minor card: no at-HEAD fast path — the
"can't change the tree" proof fails under concurrent writers, and
pending work flushes onto the branch it was made on. One sequence,
no special case; recorded in 06's Branch switching intro. No code
change.

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

DESIGN/06-history-undo.md

@@ -46,7 +46,7 @@ The pathfinder's message engine carries over as the model — it is what earns t
 
 ## Branch switching
 
-Switching (or creating-and-switching) a branch from the board popover:
+Switching (or creating-and-switching) a branch from the board popover — **create-and-switch runs the identical settle → flush → stamp → switch sequence, no at-HEAD fast path** (blessed 2026-07-31): "creating at HEAD can't change the tree" fails as a proof under concurrent writers (a foreign commit can land between the check and the create), and flushing pending work onto the branch it was made on is the honest cadence regardless — the occasional save-or-discard beat on a create buys one sequence with no special case:
 
 - **Settle the editors first — explicitly, never silently.** Branch switch neither silently commits nor silently abandons an open card-body Edit session: if any open card window has one (unsaved keystrokes, or on-disk ~700 ms saves the session hasn't committed — the mid-session state Auto-commit above deliberately leaves uncommitted), the switch presents a **save-or-discard step**: **Save All** ends every session with its normal commit (each card's Edit→Preview flip), **Discard** reverts buffers and uncommitted saves to HEAD, **Cancel** keeps the current branch and the sessions. **Open raw-source buffers are settled by the same step** (settled — an unsettled raw buffer is the worse hazard: its Apply later writes the *entire* pre-switch `index.md` byte-for-byte onto the new branch's card): Save All *applies* each raw buffer — and since Apply validates, a buffer that fails validation cancels the whole switch with focus on the offending window, nothing half-switched; Discard exits raw source without writing; Cancel keeps everything. External checkouts the app can't gate are the accepted last-writer-wins case, same as the Edit buffer (05-card-window.md's dirty-buffer rule; on git boards the overwritten version is a commit, one revert away). Silently flushing the commit alone would be wrong twice over: the tree can be clean precisely because a save hasn't landed, and a later debounced save would write old-branch text onto the new branch's card. With sessions settled, the pending auto-commit flushes (flush-before-overwrite above) and checkout runs on a truly settled tree: it cannot fail dirty, and no in-flight work is lost or dragged across branches. **The flush is Save-All-shaped** (blessed 2026-07-31): after Discard, the session's reverted bytes are *reconciled, never flushed* — the pending window for that folder drops, since disk again agrees with HEAD and committing the discarded saves would betray the button; pending changes elsewhere on the board still flush normally. (Inline title editors need no step of their own: reaching the popover's branch controls commits them — click-away commits, and board-scoped commands disable while one is focused — 04-interactions.md ▸ Grammar.)
 - **The undo/redo stack does not survive a switch.** It is discarded and reseeded from the new HEAD's first-parent ancestry — the relaunch rule applied at switch time; redo starts empty. (Replaying a restore commit from the previous branch onto the new one would be wrong.)